An overhead power line is an aerial electric power transmission line, comprising one or more bare electric conductors suspended by towers or pylons. Since most of the insulation is provided by air, overhead power lines are generally a convenient method of transmission for large quantities of electric energy and are therefore mainly used for carrying high voltage (HV) current (typically higher than 30-35 kV). The overhead power lines need towers for being supported and insulators for insulating the structure of the towers (typically lattice type steel towers) from each conductor of the overhead electric energy transmission line; the ends of the insulated cable are connected to the overhead power lines at the relevant terminal stations or substations by means of components, called terminations, suitable to provide the required transition from the bare conductor of the aerial line, insulated by air, to a cable, having a conductor covered by an insulation layer and an earthed electric screening.
Overhead power lines typically comprise substations as part of electrical generation, transmission, and distribution system. For example, substations transform voltage from high to low, or the reverse, connect the overhead power lines to cabled lines, or perform any of several other functions of the electric network.
Ordinary and extraordinary maintenance need to be performed on overhead power lines, especially in the substations, for example when refurbishment and reconstruction of tower and overhead power lines occurs; when restoration or expansion in substations is required; when power transformers, circuit breakers, switchgears, shunts, or other components of a substation are to be replaced.
Such operations are performed by disconnecting all the components involved in the maintenance activity from the electric transmission line. Temporary by-pass systems which connect input and output points within the substations are used in order to guarantee the continuity of electric power supply to the users during the maintenance period. Typically, at least one of the ends of the by-pass system, or both of them, is connected to the overhead power line.
An example of by-pass system for overhead power lines is described in HVSBL January 2006 by Silec Cable (www.sileccable.com/Portals/france/pdf/en/2151_HVSBL.pdf).
According to this publication, a three phase High Voltage Stand-By Link (HVSBL) system is composed of three lengths of cable equipped with two synthetic terminations pre-mounted in factory; three dedicated drums allowing storage, transportation and unwinding and rewinding of the lengths equipped with their terminations; six metallic structures for supporting the terminations during use (optional).
The illustrated link systems are conditioned on dedicated metallic drums fitted out to contain and protect the cable equipped with its two terminations. A three phase link system may comprise one single drum of 2.6 m in diameter equipped with three compartments allowing to install on the same drum the three phases of a 90 kV HVSBL of 20 m, to three specific drums of 4.7 m each one containing one phase of a 225 kV HVSBL of 350 m. Generally, the terminations are installed in protections (tube or under-frame) positioned and fastened on the inner side of the drum (for transportation). Termination length increases with voltage and can be comprised between 1.8 m for 90 kV and 2.8 m for 225 kV. Drums delivered, laying equipment and routine tools are transported by truck from a storage yard to the installation site. A metallic structure supporting the terminations during operation can be a metallic structure allowing the mechanical support of a termination; each termination is lifted by a crane for being positioned on the top of a respective metallic structure and mechanically coupled thereto.
Typically, the installation procedure of temporary by-pass links for a three-phase power line comprises the following steps:                unloading three link-bearing drums from a truck, by means of a crane;        unwinding a cable length of each link from each drum and lay it down to the site of connection of the outdoor termination thereof;        providing the site of connection with a supporting structure for each outdoor termination of each link;        connecting each outdoor termination to the overhead power line and the other cable end to another power line.        
Such a procedure takes a substantial amount of time (e.g. many work weeks, especially in case of termination for voltage higher than 200 kV), and this is a critical, especially in case of a line failure to be restored.
Due to the necessity of maintaining or repairing overhead power lines, by-passing certain positions or points of an electric energy transmission line is usually a mandatory operation to guarantee safe condition and to guarantee the electric power supply to the users.
The Applicant has observed that it would be advantageous to have short operating time for providing by-pass on electric energy transmission lines.
In particular, the Applicant has tackled the problem of providing a by-pass system for overhead power lines which requires shorter time and few staff for being installed in the field, compared with conventional ones.